In the field of wireless communications, multiple-input-multiple-output (MIMO) is one of the promising schemes to increase system link robustness and spectral efficiency. The basic idea of spatial diversity is that multiple antennas are less likely to fade simultaneously than a single antenna element. Diversity techniques increase the average signal-to-noise-ratio (SNR) by means of coherent combining. Space-time coding is a particularly attractive approach to realize transmit-diversity gain without requiring channel knowledge at the transmitter. Another type of diversity scheme is delay diversity, where each transmit antenna sends a delayed version of the same signal, which can be readily exploited through the use of coded orthogonal frequency division multiplexing (OFDM).
Employing multiple antennas at both ends of the wireless link can dramatically increase the bit rate using the spatial multiplexing scheme. Spatial multiplexing multiplexes a high rate signal into multiple bit-streams, and then transmits them simultaneously using multiple antennas, allowing a linear capacity increase.
However, a spatial multiplexing scheme requires a rich scattering environment. For channels that are less amenable to spatial multiplexing (e.g., for near line of sight stations, antenna correlation, etc.), spatial diversity is used where transmit-receive antenna diversity is exploited. Therefore, spatial multiplexing and spatial diversity are complementary approaches when using multiple antennas. In the TGn Sync technical specification, S. A. Mujtaba, “TGn Sync Proposal Technical Specification,” a contribution to IEEE 802.11, 11-04-0889r56, May 2005 (incorporated herein by reference), spatial multiplexing, space-time coding and delay diversity are all specified as transmission schemes. The intelligence for mode switching between spatial multiplexing, delay diversity and space-time coding located in the link adaptation module, as an extension of adaptive coding and modulation scheme, plays an important role in achieving the IEEE 802.11n system capacity.
Conventional approaches focus on designing schemes to extract either maximal diversity gain or maximal spatial multiplexing gain. L. Zheng and D. Tse, “Diversity and multiplexing: a fundamental tradeoff in multiple-antenna channels,” IEEE Trans. Info. Theory, vol. 49, May 2003, presents a fundamental tradeoff between how much each coding scheme can get. However, no detailed workable algorithm is provided by the conventional systems.